This invention relates to a method and apparatus for encoding a n-bit information word into a m-bit code word, the encoded code words exhibiting desirable characteristics, for example, for magnetic recording.
Recently, new applications have been introduced for digital recording techniques. For example, digital video tape recorders (DVTR) have been developed, wherein a composite color television signal in analog form is converted to a digital signal, and the digital video signal then is recorded directly on a record medium, such as magnetic tape. To assure accurate reproduction of the video signal, it is advantageous to encode the digital signal, prior to recording, in a format whereby inherent limitations of the magnetic recording system are avoided. One proposal for a digital VTR is described in, for example, copending application Ser. No. 192,358 now U.S. Pat. No. 4,329,708.
It is known that in magnetic recording, a DC component of the digital signal cannot be reproduced. Generally, therefore, the bit transitions of the recorded digital signals are detected, and the original digital signal is reconstructed therefrom. Since the original DC component of the digital signal is lost, it is advantageous to encode that signal in a form whereby, over a period of time, the overall DC component of the encoded signal is equal or close to zero. Various encoding techniques have been proposed to accomplish the foregoing.
In one encoding technique which has been proposed to minimize the overall DC component of the encoded digital signals, n-bit information words are represented by corresponding m-bit code words, wherein m&gt;N, and the so-called "disparity" of each m-bit code word is equal to zero. The word "disparity" as understood herein refers to the DC component of the code word. If a binary "1" is represented as the value +1 and a binary "0" is represented as the value -1, then the difference between all of the binary "1"s and "0"s which constitute a code word will represent a positive or negative number or zero, depending upon the difference between the number of "1"s and "0"s. This number is the disparity of the code word. The sum of all of the disparities of a string, or series, of successive code words is the "digital sum variation" of those words. Thus, the digital sum variation may be thought of as a running sum, or total, of the individual disparities of each code word, and represents the overall, or net, DC component of the successive code words that have been produced.
In one encoding technique, each code word is formed of a sufficient number of bits such that every n-bit information word can be represented by a respective m-bit code word having zero disparity. For example, a (4, 6; 0) coding technique operates to encode an original 4-bit information word into a 6-bit code word, each 6-bit code word exhibiting zero disparity. It will be appreciated that a 4-bit code word is capable of representing only sixteen different words, and that there are twenty zero disparity words available in 6-bit codes.
Another encoding technique wherein the digital sum variation is minimized is the so-called (8, 10) encoding. This technique is described in copending application Ser. No. 199,598, now U.S. Pat. No. 4,348,659. As described therein, of the 256 information words which are available from the 8-bit digital information words, only 252 10-bit code words exhibit zero disparity. The remaining four 8-bit information words thus are represented by those 10-bit code words whose disparity is equal to +1 or -1. The non-zero disparity code words are judiciously assigned to those information words which exhibit the lowest probability of occurrence. Moreover, each of the four 8-bit information words which must be represented by a non-zero disparity code word is represented by one positive disparity code word and one negative disparity code word. Thus, when a non-zero disparity code word of, for example, positive disparity is selected, the next-selected non-zero disparity code word exhibits negative disparity. Thus, the overall digital sum variation is minimized.
In the aforedescribed encoding techniques, and particularly, the (8, 10) encoding, the wavelength of the recorded digital signal may become too short. That is, many of the code words which will be recorded include successive bit transitions, such as 101010 . . . , resulting in a high frequency of transitions and, thus, a short wavelength. This high transition density results in a reduction in the recording density, which is a disadvantage. It is desirable, therefore, to increase the minimum wavelength of the encoded digital signal to be recorded, as by increasing the minimum interval, or separation, between successive bit transitions.
Another disadvantage of the aforedescribed encoding techniques, and particularly the (8, 10) conversion is that waveform interference may result therefrom. In the (8, 10) conversion, a 10-bit code word must be recorded in the same space as the original 8-bit information word. This means that the bit interval for each bit of the 10-bit code word is 0.8 times the bit interval of each 8-bit information word. Consequently, the minimum time interval between transitions of the 10-bit code word is equal to the bit interval thereof, or 0.8T, wherein T is the assumed bit interval of the 8-bit information word. The maximum time interval between transitions of the 10-bit code word is equal to the time duration of the word itself, or 8T. Hence, the ratio of the maximum to minimum transition time intervals (T.sub.max /T.sub.min) is equal to 10. Such a large ratio may result in waveform interference and, moreover, may deleteriously influence the self-clocking of the reproduced 10-bit code word. Still further, the detecting window during which each bit of the 10-bit code word must be detected, or sensed, in order to discriminate binary "1"s from "0"s is reduced to the bit interval of the 10-bit code word, or 0.8T. Hence, there is less time to discriminate the bits of the 10-bit code word, and this may result in erroneous decoding of the digital signals and re-conversion back to original analog form.